C 3684 (OTTO) Copper(II) oxide, 99.99% Cas no 1317-38-0 - used for blue-green pigmentation in ceramics. C 3684 (OTTO) Copper(II) oxide, 99.99% Cas no 1317-38-0 - used in the preparation of intermediates, oxidizing/reducing agents, surface treating agents and process regulators.